The use of devices which measure a change in reluctance for sensing or determining velocity are known in the prior art. In these devices, a pole piece is mounted for movement relative to a core. A magnet associated with the core produces a magnetic field, and the pole piece functions to carry the magnetic flux in a return path to the magnet. The pole piece is separated from the core by a very small gap, and as the pole piece moves relative to the core, it couples more or less flux through the return path and across the gap. This induces an electrical signal in a secondary coil, which is detected and converted into a velocity measurement. Since the flux is inversely proportional to the reluctance, and the reluctance is proportional to the area of overlap between the pole piece and core, divided by the gap, a decrease in the gap and/or an increase in the overlapped area will result in an increase in flux, and vice versa. A change in the gap or a change in the overlapped area will thus produce a signal that may be taken as representative of a change in velocity.
One of the problems associated with a conventional velocity sensor is its sensitivity to motion in more than one degree of freedom, thereby introducing error into the measurement.
Accurate sensing of angular rotation velocity relative to a base reference is particularly hard to achieve. Sensors currently in use for this purpose typically comprise a permanent magnet-resolver type of structure that requires bearings to support a shaft for rotation. These structures are delicate and expensive to build.
Accurate differential velocity sensing is even harder to achieve since it requires rotation of a shaft and a case, thus further adding to the complexity and expense of the structure.
In conventional velocity sensors, cross axis rotation of the pole piece relative to the core may cause a change in the gap, which results in a variation in the reluctance. This variation in reluctance is, in turn, erroneously sensed as a change in the velocity. In addition, translational movement of the pole piece relative to the core along an axis not intended to be a sensitive axis can cause a change in overlapped areas between the pole piece and core, thus producing a change in the reluctance.
One typical prior art velocity sensor comprises a single permanent magnet and a single coil. The problem with this design is that it usually consists of a round cylindrical shape with a permanent magnet positioned in the center of the coil. Detected flux changes occur mainly because the magnetic flux field is not uniform. If the coil is moved over the center of the magnet, the signal goes to zero because the gradient of the magnetic field goes to zero. This design does not achieve a differential measurement that is sensitive to motion in only one degree of freedom and insensitive to motion in the other five.
Applicant's above-referenced copending application Ser. No. 07/335,141 describes a position sensor that in a preferred embodiment comprises an E-shaped core 52 having a primary leg 56 and two secondary legs 58 and 60. A primary coil 54 is wound on the primary leg, and secondary coils 62 and 64 are wound, respectively, on the secondary legs. Energization of the primary coil produces an electromagnet which creates AC varying flux. Pole pieces 70 and 72 are movable in gaps between the primary leg and a respective secondary leg for controlling the amount of flux carried through each secondary leg and the amount of voltage consequently induced in its associated coil. The AC varying flux is generated at a constant frequency, and the amplitude of that frequency is measured to obtain a signal representative of the displacement or position. Stated differently, a differential displacement of the pole pieces produces a differential in the electrical signals, which corresponds to a change in position. The pole pieces and legs of the core are dimensioned so that one overlaps the other an extent to insure that relative displacement between them along axes about which motion is not intended to be sensed will not produce any differential flux. Additionally, the pole pieces are coupled together for equal but opposite motion whereby common mode errors are cancelled. This device is thus sensitive to motion only in the direction indicated by arrows 74. Several different embodiments are also disclosed, which function in essentially the same way to produce differential flux splitting and insensitivity to the other five degrees of freedom.
Applicant's U.S. Pat. No. 5,070,263 describes a torque motor for driving a multi-axis angular rate sensor. The disclosed structure is very similar to the structure of a preferred embodiment described and claimed in the present application, but the claimed invention in that application is limited to details of the torque motor. In that application, a rate sensor 10 includes a permanent magnet 30 that is magnetically coupled to an upper rotor 18 and a lower rotor 44. At three circumferentially spaced locations, the upper rotor carries an axially projecting tab 60 which is centered between two electromagnetic coils 56, 58 carried by a base plate positioned between the upper and lower rotors. At corresponding circumferentially spaced locations, the lower rotor carries two axially projecting tabs 62 that extend adjacent opposite faces of the electromagnetic coils. When no electrical current is flowing through the electromagnetic coils, a plurality of flexures 46 that connect the upper and lower rotors provide a spring bias to maintain the tab 60 centered in a slot 66 and thus equidistant between each of the two tabs 62. When an electric current flows through the electromagnetic coils, distribution of the magnetic flux produced by the permanent magnet 30 changes so that the upper tab is attracted toward one of the electromagnetic coils and repelled away from the other. At the same time, the lower tabs are respectively repelled away from and attracted toward the opposite faces of the electromagnetic coils. The upper and lower rotors are thus dithered back and forth about a central axis each time the electromagnetic coil polarity changes due to a change in the direction of electrical current flowing through the coils. Flux from the magnet is carried through an upper pole piece 24 and thence through the rotors to a lower pole piece 32 and back to the magnet. This device thus provides a bi-directional, limited angle, reactionless torque motor for driving the rate sensor.
Copending application Ser. No. 653,535 describes a structure that is essentially the same as that described in U.S. Pat. No. 5,070,263, but which is directed to the angular rate and linear acceleration sensing aspects of the device. Return paths for the flux include raised steps on the upper and lower rotors, which are coupled together for equal but opposite rotation relative to one another. The dimensional relationships of the raised steps and pole pieces are such that the flux is differentially split between the pole pieces. This application is directed to the provision of accelerometers carried by the rotors, with means to sense angular rate and linear acceleration relative to three orthogonal axes, while eliminating or cancelling common mode errors.
None of the prior art devices known to applicant teaches means for measuring velocity in only a single degree of freedom, while rejecting the other five degrees of freedom. There is, therefore, need for a velocity sensor that is easy and economical to construct, reliable in operation and sensitive in only a single degree of freedom.